Arterialpulsepressureprovidesvaluableinformationaboutaperson’sstrokevolume,providedthatarterialcomplianceisessentiallynormal.Patientswhohaveseverecongestiveheartfailureorwhohavesufferedaseverehemorrhagearelikelytohaveaverylowarterialpulsepressurebecausetheirstrokevolumesareabnormallysmall.Conversely,individualswithlargestrokevolumes,asinaorticvalveregurgitation,arelikelytohaveanincreasedarterialpulsepressure.Similarly,well-trainedathletesatresttendtohavelargestrokevolumesbecausetheirheartratesareusuallylow.Theprolongedventricularfillingtimesintheseindividualsinducetheventriclestopumpalargestrokevolume,andhencetheirpulsepressureislarge.
The radial stretch of the ascending aorta brought about by left ventricular ejection initiates a pressure wave that is propagated down the aorta and its branches. The pressure wave travels much faster (≈4 to 12 m/second) than the blood itself does. This pressure wave is the “pulse” that can be detected through palpation of a peripheral artery.
Inchronichypertension,aconditioncharacterizedbyapersistentelevationinTPR,thearterialpressure-volumecurveresemblesthatshownin
Fig.17.17B . BecausearteriesbecomesubstantiallylesscompliantwhenParises,anincreaseinTPRcausessystolicpressuretobemoreelevatedthandiastolicpressure.Diastolicpressureiselevatedinsuchindividualsbutordinarilynotmorethan10to40mmHgabovetheaveragenormallevelof80mmHg.Notuncommonly,however,systolicpressureiselevatedby50to100mmHgabovetheaveragenormallevelof120mmHg.
Thevelocityofthepressurewavevariesinverselywitharterialcompliance.Ingeneral,transmissionvelocityincreaseswithage,whichconfirmstheobservationthatthearteriesbecomelesscompliantwithadvancingage.Velocityalsoincreasesprogressivelyasthepulsewavetravelsfromtheascendingaortatowardtheperiphery.Thisincreaseinvelocityreflectsthedecreaseinvascularcomplianceinthemoredistalportionsthaninthemoreproximalportionsofthearterialsystem.
ThePacontourbecomesdistortedasthewaveistransmitteddownthearterialsystem.Thisdistortioninthepressurewavecontourofthehumanarterialtreeisdemonstratedasafunctionofageandofrecordingsitein
Fig.17.19 .Dampingofthehigh-frequencycomponentsofthearterialpulseiscausedlargelybytheviscoelasticpropertiesofthearterialwalls.Thepulsepressurewavetravelsmorerapidlyinolderpeoplethanintheyoungerpeople,asaconsequenceofreducedcompliance.Severalfactors—includingwavereflectionandresonance,vasculartapering,andpressure-inducedchangesintransmissionvelocity—contributetopeakingofthePawave.
Most commonly, blood pressure is estimated indirectly by means of a sphygmomanometer. In hospital intensive care units, needles or catheters may be introduced into the peripheral arteries of patients to measure arterial blood pressure directly by means of strain gauges. When blood pressure readings are taken from the arm, systolic pressure may be estimated by palpation of the radial artery at the wrist (palpatory method). While pressure in the cuff exceeds the systolic level, no pulse is perceived. As pressure falls just below the systolic level (
Fig. 17.20A ), a spurt of blood passes through the brachial artery under the cuff during the peak of systole, and a slight pulse is felt at the wrist.